Problem: It takes 30 minutes for 8 people to paint 8 walls. How many minutes does it take 12 people to paint 12 walls?
Explanation: Imagine that each person is assigned one wall, and all 8 people begin painting at the same time. Since everyone will finish painting their assigned wall after 30 minutes, it takes one person 30 minutes to paint one wall. If we have 12 people and 12 walls, we can again assign one wall to each person. Everyone will take 30 minutes to paint their assigned wall. In other words, it takes 30 minutes for 12 people to paint 12 walls.